1. Field of the Invention
The present invention relates to fiber optic probes for spectrophotometric analyses. In particular, the present invention relates to a fiber optic probe for attenuated total internal reflection spectroscopy.
2. Discussion of Background
The development of improved optical fibers, multichannel array-type spectrophotometers and multiplexing technology has led to increased use of remote spectroscopic techniques for in-line monitoring and process control, environmental monitoring, and medical applications. Signal transmission via optical fibers allows for the placement of sensitive equipment in central locations, making remote sensing a particularly attractive choice for monitoring processes that take place in harsh industrial process environments. In the environmental field, remote sensing techniques are used for in situ measurements of fluids in wells, boreholes, storage and process tanks, etc. Applications include monitoring groundwater flow, studying the migration of subsurface contaminants, evaluating the progress of remediation operations, and detecting toxic or explosive substances. Fiber optic probes can be used with absorption, diffuse reflectance, and Raman spectroscopy.
Optical analysis techniques also improve the quality of the data. Data obtained from a sample are not always truly representative of the source of that sample, since the mere act of taking the sample can alter its properties; frequently, removing a sample can perturb the source as well. Optical techniques can frequently be implemented without the need to take samples for laboratory analysis elsewhere; therefore, data from optical analyses can be more reliable than data obtained using other analytical techniques.
Absorption or transmission spectrophotometry is perhaps the most versatile and widely used optical analysis technique. The absorbance of a sample is defined as A=-log.sub.10 T, where T=I/I.sub.0, I is the intensity of the light transmitted light transmitted through the sample, and I.sub.0 is the incident light intensity. The amount of light absorbed by the sample at different frequencies depends on the concentration of each constituent. Therefore, the absorption spectrum of the sample--the frequency distribution of the absorbance-can be used to identify its composition.
Absorption spectroscopy requires samples that are optically translucent or transparent in the range of frequencies being studied. Therefore, conventional absorption spectroscopy is difficult or impossible for analysis of dark or opaque samples, including inks, dye baths, and other extremely dark fluids. Other techniques based on analysis of the light scattered by the sample, such as diffuse reflectance, fluorescence, total internal reflectance, and Raman spectroscopy, are useful for in situ analysis of dark and opaque liquids, solids or slurries. In probes designed for these types of measurements, light is directed to the sample through a transmitting fiber; scattered or reflected light is collected by the receiving fiber and returned to the detector.
Attenuated total internal reflection spectrophotometry (also known as "ATR" or "ATIR" spectrophotometry) is a useful technique for analyzing dark and opaque liquids and slurries. ATIR is a consequence of internal reflection at the interface between two media having different refractive indices. When light passing through a medium of high refractive index strikes an interface with a medium of low refractive index, total internal reflection occurs if the angle of incidence is greater than the critical angle (wherein the term "critical angle" refers to the angle of incidence above which total internal reflection occurs). That is, the critical angle is that angle r for which sin(r)=n.sub.i /n.sub.r, where n.sub.i and n.sub.r are the refractive indices of the two media with n.sub.i &lt;n.sub.r. The attenuation (the decrease in intensity) of the light beam on reflectance is proportional to the change in refractive index between the two media at the interface. Because the refractive index tends to change markedly near absorption bands, the ATIR spectrum of a substance is similar to its absorption spectrum. In general, the ATIR spectrum of a sample is independent of the thickness of the sample, but varies depending on the angle of incidence of the incident light. The smaller the angle of incidence, the greater the penetration into the sample-however, the angle of incidence must be greater than the critical angle for total internal reflection to occur.
A problem that is commonly encountered in ATIR spectrophotometry is the low intensity of the reflected light compared to the intensity of the incident light (the term "attenuated" refers to the common practice of placing an attenuator in the incident beam to balance the energies of the incident and reflected beams). Chemometric techniques may be useful to help factor out background noise and identify the signal of interest.
Like absorption spectrophotometry, ATIR spectrophotometry requires a light source, an optical probe with light-transmitting and light-receiving fibers, and a detector. Suitable probes have a high-index optical element interposed between the fibers and the sample. The element must not only have an index of refraction that is higher than that of samples to be measured, but it must be durable and impervious to the sample constituents. To maximize the light-gathering capacity of the light-receiving fiber, the fibers must be very precisely aligned to optimize the angle of incidence of the light striking the interface between the optical element and the sample. Typical ATIR probes are complex and delicate, and are not well suited for use in many environmental, medical, and industrial process environments. In part because of these problems, ATIR spectrophotometry has been largely confined to research laboratories.
A wide variety of fiber optic probes are available for use with spectrophotometry systems. By way of example, U.S. Pat. No. 5,168,367 describes a variable path length probe for spectrophotometric measurements of fluids in situ. For Raman-type measurements of scattered light, probes designed for improved light coupling efficiency, such as that described in U.S. Pat. No. 5,402,508, are desirable. The probe includes a housing with a transparent window across its tip for protecting the transmitting and receiving fibers. The endfaces of the fibers are slanted, forming a beveled tip that improves light coupling efficiency between the transmitting and receiving fibers. A device for making in situ optical measurements in boreholes, wells, and the like has a support structure bearing one or more probes, each probe disposed at an acute angle (U.S. Pat. No. 5,335,067). A self-referencing probe for in situ optical absorption measurements is shown in U.S. Pat. No. 5,039,224. Additional probe designs are disclosed in the following commonly-assigned, co-pending applications: Ser. No. 08/676,432 (Fiber Optic Probe), filed Jul. 8, 1996; USPS Express Mail Label No. EM 038050777US (Fiber Optic Probe System for Spectrophotometric Analyses), filed herewith; USPS Express Mail Label No. EM 038050735US (Fiber Optic Raman Probe and Coupler Assembly), filed herewith; and USPS Express Mail Label No. EM 038050752US (Retro-Reflecting Probe and Collimating Lens Assembly), filed herewith. The disclosures of the above-referenced patents and patent applications are incorporated herein by reference.
Despite the availability of a variety of fiber optic probes for diverse applications, there is a need for a simple, rugged, inexpensive and easy-to-manufacture probe for ATIR spectrophotometry. Such a probe would further the use of ATIR measurements for on-line monitoring in a wide range of laboratory, medical, environmental, and industrial environments.